1. Field of the Invention
This invention relates to new and improved chemical processes for preparing 1,1-dihalo-4-methylpentadienes, intermediates in a known method for the production of certain pyrethroid insecticides.
2. Description of the Prior Art
Pyrethroids, naturally-occurring and synthetic derivatives of cyclopropanecarboxylic acid, have long been of interest as insecticides because they are active against a wide range of insect species, they display relatively low toxicity toward mammals, and they do not leave harmful residues. A notable recent technical advance in the pyrethroid art was the discovery of dihalovinylcyclopropanecarboxylates, such as 3-phenoxybenzyl 2-(.beta.,.beta.-dihalovinyl)-3,3-dimethylcyclopropanecarboxylates, having an outstanding combination of insecticide properties [Elliott et al., Nature, 244, 456 (1973); ibid., 246, 169 (1973); South African 73/3528]. Since Elliott's discovery, a great deal of interest has been generated worldwide in economical processes for the production of this type of pyrethroid.
Several years before Elliott's discovery, a method for the production of ethyl 2-(.beta.,.beta.-dichlorovinyl)-3,3-dimethylcyclopropanecarboxylate was reported [Farkas et al., Coll. Czech. Chem. Comm., 24, 2230 (1959)]. The latter leads to an Elliott pyrethroid by ester interchange [Nature, 244, 456 (1973)]. According to the Farkas method, ethyl 2-(.beta.,.beta.-dichlorovinyl)-3,3-dimethylcyclopropanecarboxylate may be prepared from chloral and isobutylene as follows: ##STR1##
The overall conversion of readily available isobutylene to 1,1-dichloro-4-methyl-1,3-pentadiene, the key intermediate for the diazotization step, is reportedly less than 40%. Furthermore, for every kilogram of 1,1-dichloro-4-methyl-1,3 -pentadiene produced, more than a kilogram of zinc dust is consumed. In a recent year, U.S. producers alone sold about 300 million kilograms of synthetic organic insecticides [Chemical and Engineering News, July 28, 1975, p. 19]. If the Elliott pyrethroid becomes a major agricultural commodity, commercial production of 1,1-dichloro-4-methyl-1,3-pentadiene by the Farkas method would seriously tax the world supply of zinc. A process leading from the mixture of alcohols produced in the first step to the pentadienes produced in the third step of the Farkas route, without consuming zinc, would be advantageous.
It has been known since at least the turn of the century that a chlorine atom bonded to carbon can be displaced in an electrochemical process. For example, symmetrical dichloroethylene can be prepared in 80% yield by the reduction of symmetrical tetrachloroethane at a copper cathode in an aqueous catholyte containing 10% zinc chloride [Brockman, "Electro-organic Chemistry," John Wiley & Sons, Inc., New York, N.Y., 1926, p. 334]. In that reaction, not only are chlorine atoms displaced, but a double bond is introduced. A more recent reference suggests that that reaction really involves the chemical reduction of symmetrical tetrachloroethane by a layer of zinc sponge which is formed on the cathode surface; the electrochemical process is simply the regeneration of the metallic zinc [Tomilov, et al., "Electrochemistry of Organic Compounds," Halsted Press, New York, N. Y., 1972, p. 282].
It is stated in the prior art that cathodic dehalogenation is conducted in protonating media, generally in acid solution, usually in aqueous sulfuric acid or hydrochloric acid [Tomilov, et al., loc cit, p. 284]. Because of the insolubility of many organic halides in water, their reduction may be carried out in a mixture of water and an organic solvent [M. R. Rifi in Baizer, "Organic Electrochemistry," Marcel Dekker, Inc., New York, N. Y., 1973, p. 301].
The electrochemical reduction of a 1,1,1-trihalo compound carrying a 2-hydroxy group has been reported [Tomilov, et al., loc cit, p. 290]. 2-(1,1,1-Trichloro-2-hydroxypropyl)pyridine was reduced to the corresponding 1,1-dichloro-2-hydroxy compound. Nowhere is it suggested in the prior art that a reductive elimination, both the displacement of a halogen atom and the introduction of a double bond, may occur in a single electrochemical process unless the compound contains vicinal halogen atoms.